DISSOCIATION OF THE $Eu^{+2}$ CHARGE-TRANSFER STATE INTO $Eu^{+2}$ AND A FREE HOLE IN Y AND La OXYSULFIDES

Abstract

Excitation into the $Eu^{+2}$ charge-transfer states (CTS) in $Y_{2} O_{2} S$ and $La_{2} O_{2} S$ leads partially to long-time energy storage or to loss, in addition to feeding the emitting $^{3} D$ states. We have explained both storage and loss as commencing with the dissociation of the $Eu^{+3}$ CTS into $Eu^{+2}$ and a free $hole.^{1}$ The amount of storage produced and also the rise-time transient response to excitation are both measures of CTS dissociation and give activation energies for dissociation of $1200 cm^{-1}$ in $Y_{2} O_{2}S, 1500 cm^{-1}$ in $La_{2} O_{2}S$. At room temperature, 1/5 of $La_{2} O_{2} S$ CTS excitations and 2/5 of $Y_{2} O_{2} S$ excitations dissociate. Losses prominent at high $Eu^{+3}$ content produce non-linearities, which are interpreted as showing that the loss is proportional to the product of $Eu^{+2}$ and trapped hole concentrations. We believe that the losses are due to the extra electrons at the $Eu^{+2}$ centers migrating through the Eu activators and recombining non-radiatively at the hole centers.